This invention relates to a planetary gear apparatus including an inner gear, planetary gears and a sun gear.
In general, the planetary gear apparatus includes an inner gear and a sun gear whose axes are aligned with the rotation axis, a carrier rotatable about the rotation axis, and a planetary gear rotatably received in receiving a hole formed in the carrier. The planetary gear is in engagement with the inner gear and the sun gear. Accordingly, when the carrier, for example, is driven for rotation, its rotation is transmitted to the inner gear and the sun gear through the planetary gear. In this case, when the planetary gear is rotated about its own axis, the inner gear and the sun gear are differentially rotated. When the planetary gear is not rotated about its own axis, the inner gear, the carrier, the planetary gear and the sun gear are integrally rotated (see Japanese Patent Application Laid-Open Nos. H04-312247, H09-112657 and H09-144844).
In the above-mentioned planetary gear apparatus, when the planetary gear is rotated about its own axis at the time of differential rotation, a friction resistance (friction torque) for stopping the rotation of the planetary gear is generated between the outer peripheral surface of the planetary gear and the inner peripheral surface of the receiving hole. Moreover, since the inner gear, the planetary gear and the sun gear have helical teeth, a thrust force acts on the area between the inner gear and the planetary gear and between the sun gear and the planetary gear. By this thrust force, the inner gear and the sun gear are pushed in the axial direction so that their end faces are abutted with a housing of the planetary gear apparatus and the end face of the planetary gear is abutted with the bottom surface of the receiving hole. As a result, a friction torque for stopping the rotation of the inner gear, the sun gear and the planetary gear is generated at the respective end faces of those gears. By this friction torque, the differential rotation between the inner gear and the sun gear is limited. Moreover, the differential limiting force varies in accordance with the input torque. Accordingly, the above-mentioned planetary gear apparatus substantially has a toque sensitive differential limiting mechanism.
However, the conventional planetary gear apparatus having such a differential limiting mechanism is encountered with a problem in which a torque bias ratio, this being a ratio between the rotation torque transmitted to the inner gear and the rotation torque transmitted to the sun gear at the time of differential rotation, fluctuates extensively.
That is, in the conventional differential gear apparatus, the teeth of the inner gear, the sun gear and the planetary gear are helical teeth so that a friction torque acts on the respective end faces of the inner gear, the sun gear and the planetary gear. Owing to the helical teeth, the places where the planetary gear is engaged with the inner gear and the sun gear are axially shifted in accordance with the rotation of the planetary gear, cyclically. When the engaging place is located on one end side of the planetary gear, the planetary gear is slanted toward one side with respect to the axis of the receiving hole by the engaging reaction between the planetary gear and the inner gear and sun gear. When the engaging place is located at the central part in the axial direction of the planetary gear, the planetary gear is in parallel with the axis of the receiving hole. When the engaging place is located on the other end side of the planetary gear, the planetary gear is slanted toward the other side with respect to the axis of the receiving hole. As the slanting state of the planetary gear changes, the contact pressure between the outer peripheral surface of the planetary gear and the inner peripheral surface of the receiving hole varies in accordance with the engaging cycle (=360 degrees/number of teeth of the planetary gear) of the planetary gear. As a result, the friction torque acting on the planetary gear cyclically fluctuates.
If the number of planetary gears is only one, the problem is not very serious because the amount of variation of the friction torque is comparatively small. However, the planetary gear apparatus normally includes a plurality of planetary gears. Presuming that the number of planetary gears is six and that the friction torque of each planetary gear varies in a range of from 0 to 2, the friction torque of the six planetary gears, as a whole, varies in a range from 0 to 12 as shown in FIG. 8. Because, in the conventional planetary gear apparatus, the friction torque of each planetary gear varies at the same phase, the variation of friction torque of each planetary gear is accumulated. This means that the range of fluctuation is so large as 6 times, that is, the range of fluctuation becomes so large as the normal range multiplied by the number of planetary gears. As a result, the torque bias ratio fluctuates extensively. This could be the cause of noises and vibrations.